This invention relates to the field of advanced transonic airfoils for flight vehicles. More particularly, the invention relates to an airfoil trailing-edge design to improve airfoil effectiveness in terms of increased thickness, increased lift capability, and decreased drag.
The aerodynamic drag of modern transonic airfoil sections consists of two components. The two components are skin friction related drag and compressibility drag. At lower speeds (Mach numbers) the airfoil section drag consists only of the skin friction related drag. As the speed or Mach number is increased, shock waves appear on the airfoil surface. These shock waves cause increased drag and are the major portion of the drag referred to above as compressibility drag. Compressibility drag increases dramatically with increasing Mach number and strongly limits airfoil efficiency in terms of lift-to-drag ratio. For example, modern air transport wing designs are developed to delay the onset of this drag rise until the design cruise Mach number is reached. The aerodynamicist utilizes both wing sweep and airfoil section characteristics as the primary variables in achieving a sufficiently high drag rise Mach number while also attaining high lift and low drag.
Recent developments in airfoil section transonic efficiency have focused upon the so-called "Supercritical Airfoil" developed by R. T. Whitcomb (U.S. Pat. No. 3,952,971). This type of airfoil section makes use of relatively flat upper surface curvature and a high level of aft-camber to achieve high lift and low drag at high Mach numbers. However, detailed design studies utilizing such supercritical or aft-loaded airfoils have revealed several adverse characteristics. First, highly aft cambered airfoils tend to be thin in the region of the wing flap structure. This thinness causes structural design difficulties with the flap system. Second, adverse, viscous boundary layer effects have been found to be more significant for highly aft-loaded airfoils. A significant amount of the aft-camber is effectively lost due to viscous boundary layer decambering near the upper surface trailing edge and in the cove region of the lower surface. As a result of these adverse characteristics, the full theoretical benefit of the so-called "Supercritical Airfoil" is not obtained in practice.
It is recognized that prior efforts have been made to increase camber in airfoil design. All trailing edge devices such as flaps as taught by Zaparka in the 1935 U.S. Pat. Re. No. 19,412 and wedges as taught by Dadone in U.S. Pat. No. 4,314,795 issued in 1982 and wedges, again, as taught by Boyd in the 1985 U.S. Pat. No. 4,542,868 have been used to change airfoil section lift. However, all of these devices produce surface discontinuities which produce earlier boundary layer separation, drag penalties and the loss in camber effectiveness resulting from these discontinuities.
It is an object of this invention to produce increased camber effectiveness in airfoil design while avoiding the surface discontinuities associated with the prior designs.
It is a further object of this invention to provide for a thicker airfoil section in the region where the flap spar is normally located which supports the trailing edge flap.